I don't think it can be always hidden by default. Go to Settings>navigation bar and there you can turn on the 'Auto hide/show the Nav bar. Then you can just swipe up from the bottom and it will hide. Swipe up again to unhide. Some apps will auto hide the navigation bar, but this has to be put in by the developer of the app.

Ok. I realize I didn't thoroughly read OP question and now see it was in reference to wp10 build. Sorry bout that. I answered via 'New Posts' and didn't pay attention that it was posted in Windows 10 Mobile section.